In the currently used nuclear power plants, the nuclear fuel has to be replaced periodically. During this replacement of fuel, other major sections are checked for maintenance. During the checking and maintenance, a particularly important thing is that the steam generator tube is checked, so that it can be confirmed as to whether there is a leaking tube or a foreseeable leaking tube. Thus the leaking of tubes is prevented before entering into the next normal operation of the atomic reactor. In carrying out the checking of the steam generator tube, there is a pre-step to be undergone. That is, as shown in FIG. 1, the level of the cooling water for the atomic reactor is lowered, and then, there is installed a nozzle dam in the entrances of a hot leg and a cold leg.
However, even under this condition, the residual heat is continuously released from the atomic reactor, and therefore, this heat has to be eliminated. Such a low water head operation is called a mid-loop operation. For example, in the Korean atomic reactors such as No 3 and 4 of Youngkwang, and No. 3 and 4 of Ulchin, the discharge hole of the steam generator is not high enough relative to the hot leg. Therefore, the maximum extra height of the hot leg in which the residual heat removal outlet lies reaches only a height which is only one half of the diameter of the hot leg. Therefore, the water head becomes insufficient in the opening of the residual heat removing system, and air is introduced to induce cavitations, with the result that the pump for the residual heat removing system is lost.
The residual heat removing pump which is used in nuclear power plants has a capacity of 4000 GPM. Below 3000 GPM, the pump shows a rumbling phenomenon, and therefore, the flow rate is maintained above the level which is just necessary for removing the residual heat. Consequently, even under an insufficient water head in the hot leg during the mid-loop operation, a flow rate of over 3000 GPM is required, thereby increasing the possibility of air suction.
For this reason, the Palo Verde atomic reactor which is the original model of the atomic reactors of Youngkwang and Ulchin of Korea could not carry out the mid-loop operation so far, in spite of the fact that a permission for the mid-loop operation had been obtained from the NRC. In the economic aspect, it is reported that there is a significant difference between the case of carrying out the mid-loop operation and the case of not carrying out it. Even by referring to the domestically applied nuclear refueling program, it is found that the maintenance checking period is increased by about 15 days or more, if the mid-loop operation cannot be carried out.
Further, as shown in FIG. 2, according to the analysis of B & W company, in the case of the Palo Verde atomic reactor, if the steam generating nozzle dam can be installed when the used nuclear fuel is taken out and when the nuclear fuel is filled after opening the head of the reactor, then the repair period can be shorted by about 12-15 days.
ABB-CE installs a single nozzle dam instead of the generally used double nozzle dam in order to install the nozzle dam just after lifting the head of the atomic reactor, so that the water head within the suction hole of the residual heat removing system in the hot leg should be heightened. If the method of ABB-CE is used, a low water level alarm can heighten the water head by 2" from 101' 5" to 101' 7", while a low low water level alarm can heighten the water head by 1" from 101' 4" to 101' 5". Therefore, the nozzle dam can be installed so much faster, while the damage due to the air introduction can be excluded during the mid-loop operation.
However, even in such a method, the flow rate of the pump has to be maintained at the normal level, and therefore, even if the water head is raised by 2", if waves are formed on the water, the air introduction cannot be prevented.
Further, according to the domestically performed experiment, it is certain that the air introduction cannot be prevented with the increase of the water head by 2" at the mid-loop.